So-called two component system heat-sensitive recording materials which utilize the color reaction of a colorless or light-colored electron donor dye precursor and an electron acceptor compound are disclosed in Japanese Patent Publication Nos. 14039/70 and 4160/68. Two component coloring system heat-sensitive recording materials are prepared by applying to a support a mixture of a fine dispersion of a colorless or light-colored electron donor dye precursor, an electron acceptor compound, a binder and the like in such a manner that the two heat-reactive compounds are kept separated by the binder and the like. When heated, one or both of the two compounds are melted and brought into contact with each other whereupon they undergo a color reaction which is used for recording. These two component coloring system heat-sensitive recording materials are advantageous and practical in that their primary coloring mechanism eliminates the need for development, that their paper quality resembles that of normal papers, that they are convenient to handle, that the color density is high, and that heat-sensitive recording materials having various color hues can be easily prepared. Therefore, these heat-sensitive recording materials are most widely used as heat-sensitive recording materials. In recent years, these heat-sensitive recording materials have shown a remarkable increase in use with facsimile, recorders and printers. As facsimiles have been used more and more, the recording speed has been improved. This has required the heat-sensitive recording material to color with shorter pulses, i.e. lower energy, that is, to have an improved heat-response.
On the other hand, these heat-sensitive recording materials are disadvantageous in that the primary coloring mechanism is such that a reaction of the electron donor dye precursor and the electron acceptor compound also occurs due to solvents or the like even without applying heat.
This is because all heat-sensitive recording materials of this type are made of organic materials which are highly soluble in solvents and thus cause a reaction therein. Accordingly, when solvents such as water ink pen, oil ink pen, fluorescent pen, diazo developer, adhesives, or paste are brought in contact with stationery, the background having white color of the heat-sensitive recording material may color or printed portions thereof may discolor, thereby impairing commercial value.
Attempts have been made to provide a solvent-resistant protective layer on the heat-sensitive coloring layer as disclosed in Japanese Patent Publication No. 27880/69, and Japanese Patent Application (OPI) Nos. 30437/73 and 31958/73 (the term "OPI" as used herein means an "unexamined published application"). However, such a protective layer lowers the coloring sensitivity. Such a protective layer also makes the recording material insufficiently compatible with the heat-sensitive head of the heat-sensitive recording apparatus, causing sticking or noises upon recording. Moreover, the protective layer makes the recording material poor in writing quality as stationery, causing ink run or the like.